<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>数据类型</title>
  </head>
  <body>
    <script>
      alert(typeof 3);

      alert(typeof "A"); //string

      alert(typeof null); //object

      var a;
      alert(typeof a); //undefined
    </script>
  </body>
</html>
